A flying thin-film magnetic head is ordinarily provided with a write element (an induction-type electromagnetic conversion element) and a read element at the air flowing-out end of the slider. The induction-type electromagnetic conversion element is covered with a protective film. The protective film, which is made of an inorganic insulating material such as alumina, forms the most outside layer at the air flowing-out end.
The induction-type electromagnetic conversion element comprises first and second magnetic films, a gap film, a coil film, an insulating film and the like. In the first and second magnetic films, the end portions at the air bearing surface (hereinafter, referred to as ABS) of the slider face each other over a gap film, so as to form a write pole portion.
And the second magnetic film extends to the rear of the ABS as keeping a space between the first and second magnetic films, and is coupled with the first magnetic film at the rear joint portion.
The coil film is wound in a spiral shape around the rear joint portion, passing through a space (inner gap) between the first and second magnetic films. Both ends of the coil film are led to the outside via terminal conductors (bump). The insulating film fills up the inner gap between the first and second magnetic films. The coil film is buried inside this insulating film. And the second magnetic film is formed on the surface of the insulating film.
In the thin-film magnetic head described above, a write current is supplied to the coil film of the induction-type electromagnetic conversion element to write information on the magnetic disk.
In supplying the write current to the coil film of the induction-type electromagnetic conversion element, the coil film generates heat due to Joule effect. Since the coil film is buried inside the insulating film of an organic insulating material or the like and the whole including the insulating film is covered with the protective film of alumina or the like, the coil film has a poor dispersion of the heat generated therein. Consequently, the heat generated in the coil film is confined inside, which causes thermal expansion of the coil film and the insulating film.
In addition to this heat generation of coil, there is a problem of heat generation due to the eddy-current loss in the core. The heat generation due to the eddy current loss causes the thermal expansion of the magnetic film, and the heat generation increases as the frequency of the current becomes higher. Consequently, a measure to counter this problem is a very important problem at the present time when a write current tends to be made higher and higher in frequency to cope with the speedup of transfer rate.
The protective film is pushed out and swollen out to the outside due to the thermal expansion force of these coil film, insulating film and magnetic film. And the swelling-out of the protective film reaches the ABS also and the ABS results in being swollen out in an area in which the induction-type electromagnetic conversion element exists. And due to the above-described heat generation, the first and second magnetic films expand thermally and their pole portions pressed by a weaker force at the ABS side are swollen out. The amount of swelling-out of them sometimes reaches a height of 10 nm.
In a magnetic disk device with this kind of thin-film magnetic head, the flying-height between the magnetic disk surface and the ABS of the thin-film magnetic head has been gradually reduced to be 40 nm, 30 nm, 20 nm or 10 nm in order to meet the demand for high-density recording. Thus, a protrusion in the ABS as described above is liable to bring a head crash, damage of a magnetic disk, destruction of magnetic recording data and the like, and comes into a very serious problem from the viewpoint of reliability of a magnetic disk device.
Another problem attendant to the heat generation is a failure generated at a part to be the forefront in recording. That is to say, as known publicly, the write current supplied to the thin-film magnetic head is composed of a preamble portion, a sink mark portion and a user data portion in each sector of magnetic recording. These are serial data arranged in time series.
In the forefront part immediately after the start of write operation, however, it is not ensured to secure a sufficient thermal protrusion and satisfactory write characteristic beginning in the forefront of the user data portion. Thus, errors are liable to happen.
In order to solve the above-described problems, various means have been proposed up to now. These means are classified into a type of suppressing thermal protrusion and a type of positively utilizing thermal protrusion as admitting it. A type of suppressing a thermal protrusion includes the following approaches:    (1) an approach of reducing the amount of generated heat by reducing the resistance value of a coil;    (2) an approach of improving the effect of heat radiation by reducing the thickness of an alumina film and the like existing under a write element;    (3) an approach of reducing an eddy-current loss by making a core small; and the like.
However, any of these approaches cannot provide a sufficient effect due to restrictions caused by the structure of a magnetic head.
As a type of positively utilizing thermal protrusion, as seen in the specification of U.S. Pat. No. 5,991,113, there has been known a technique of producing thermal protrusion before a write operation by burying a heater member in a protective film. However, this structure cannot avoid a problem of needing to have a heater member buried in particular.
JP 4-366,408A has disclosed a technique of providing a depression in advance in the medium facing surface of an inorganic insulating protective film. This depression is formed in which the medium facing surface of the protective film is made swollen out in advance in consideration of the amount of protrusion caused by thermal expansion of the insulating film and a flattening process is performed on the medium facing surface.
This prior art is based on an idea in that the protrusion caused by the thermal expansion is admitted, not suppressed and the portion to protrude is simply made retreated in advance to prevent the protrusion of ABS due to thermal expansion. Consequently, the prior art does not provide a means of completely solving the protrusion due to thermal expansion.
As described above, since it is difficult to completely suppress the protrusion caused by thermal expansion, the prior art has taken a technique in that the protrusion caused by thermal expansion is admitted and the flying-height is increased to a height value enough for the prevention of head crash. This impedes high-density recording.